The immune system--the body's defense against disease--is one of the most complex systems in the horse, consisting of an elaborate set of cells and cell mediators that respond to external factors such as stress, exercise, stage of life, and challenges from pathogens. Both humans and horses must consume nutrients to support all body systems, including the immune system. But how do we know which nutrients support the immune system, and how do they work?

Mechanisms of Immunity and Nutrition

Nutrition and immunity interact in a variety of ways, many of which researchers are just beginning to understand. In a book titled Scientific Advances in Animal Nutrition, Kirk C. Klasing, PhD, summarizes the mechanisms of nutrition and immunity. These include:

Nutritional immunity The body can fight infections by withholding nutrients from pathogens; this concept is called nutritional immunity. For example, iron, biotin, and manganese play a role in depriving pathogens of required nutrients.

Hormonal regulation Certain vitamins, minerals, and macronutrients (substances required in large amounts) provide the molecules needed for hormonal regulation. Glucocorticoids (a group of steroid hormones) are essential for mediating the body's response to pathogens. Hormones also regulate the activities of the lymphocytes (white blood cells capable of responding to the presence of foreign material in the body; they play a central role in coordinating immune responses).

Fatty acids and vitamins A, D, and E These all have direct effects on the immune system by supporting the manufacture and function of cytokines--the chemical signaling molecules of the immune response.

Substrates These provide the molecules required for metabolism. All nutrients provide the amino acids and energy that help build and maintain the immune system components and their metabolic processes. For example, vitamin A helps with the development of B- and T-lymphocytes (key parts of the immune response).

Reduction of pathology (diseased tissue) induced by the immune response "There are certain nutrients that we know are important for immune function," says Kathleen Crandell, PhD, an equine nutritionist at Kentucky Equine Research. "We know, for example, that antioxidants, particularly vitamin E and selenium, are important for support of the equine immune system."

Oxidation (the loss of oxygen atoms) This is a chemical process that occurs as a normal part of metabolism, as well as in disease processes such as inflammation and infection. Antioxidants are molecules that reverse the chemical process of oxidation by stabilizing the number of unstable oxygen atoms (free radicals) that can cause cell damage over time. At the molecular level oxidation that free radicals cause can damage DNA, RNA, and the cell lipids (fat molecules that make up the cell membrane), which are essential for cell membrane integrity. Weakening of the lipid layers can hasten cell death. Important antioxidants are vitamins E, C, and A, and minerals such as zinc and selenium, as mentioned above.

Vitamins A and E in particular have antioxidant effects in the horse, adds Crandell. For instance, vitamin A might direct lymphocyte development and maintenance of the mucous membranes, which help fight off challenges from pathogens.

Feed Design, Nutrition, and Immunity

How do we know if the feed we give our horses has the right amounts of nutrients for healthy immunity?

"When you design a feed, you are not just looking at immune function, you are looking at overall nutrition requirements," says Crandell. And, according to a fact sheet by Sarah Ralston, VMD, PhD, Dipl. ACVN, of the Department of Animal Science at the Rutgers University Equine Science Center, "The most common nutrient concerns when balancing rations are the water, energy, fiber, protein, calcium, and phosphorus content of the feed."

Ralston has this to say about analyzing vitamin levels in feed: "Vitamin assays are much less available than are mineral assays. The only vitamin routinely measured at commercial laboratories is vitamin A. Usually, assays for vitamin A measure total carotenoids, which will overestimate the actual amount of vitamin A activity, but at least gives an idea of the vitamin value of the feed. With increasing use of high- performance liquid chromatography, assays for vitamin C, B vitamins, and vitamin E may become more available. Biologic assays, using either animal or bacterial response to extracts of the feed or substance in question, also are used to determine concentrations of most vitamins. These assays are lengthy and expensive (because they require a level of sophistication usually found only in research laboratories)."

Crandell notes, "The amounts of nutrients that will go into a feed will be those that meet the optimal requirements for normal body function for a certain level of activity, such as a performance horse or a young growing horse, but they are not necessarily designed specifically for improving immune function. If you have a balanced diet, then you will be supporting the immune system."

When a manufacturer makes the claim that their product boosts the immune system, they are usually referring to the presence of antioxidants.

NRC Guidelines

When determining the amounts of nutrients needed, feed manufacturers start with the National Research Council (NRC) report Nutrient Requirements for Horses. The NRC is part of the National Academy of Sciences, a private nonprofit research institution that provides science, technology, and health information that helps officials build policy and industry guidelines. The NRC equine committee gathers, analyzes, and publishes information about research in equine nutrition. This information can be found on its Web site, www.nap.edu.

The last NRC was released in 2007, and it had not been updated since 1989. It includes: general energy requirements of horses; guidelines for nutrition and exercise; chapters on carbohydrates, fatty acids, vitamins, and water requirements; feeds and feed processing; guidelines for unique aspects of nutrition, such as feeding orphan foals; interactions between feeding management and several disorders, including colic, laminitis, recurrent airway obstruction, PSSM (polysaccharide storage myopathy), and gastric ulcer syndrome; and the report contains a section on the latest evidence for dietary supplements.

The NAS provides a free Web-based calculator that allows users to calculate nutrient requirements for individual horses according to the NRC guidelines. This can be found at http://nrc88.nas.edu/nrh.

"The NRC information is intended as a guideline only and does not necessarily represent the optimal requirements," says Crandell. "The council examines as many studies as it can find on specific nutrients, and compiles guidelines from the results.

"There is a sort of gray zone between the minimal requirements and the optimal requirements," she adds. "For example, the selenium levels for optimal immune function could be greater than those required to prevent classical signs of deficiency."

The cutting-edge feed manufacturers go for what they deem are the optimal nutrient amounts. In the case of selenium, the amounts might be two or three times what the NRC guidelines recommend.

Vitamins and Minerals

Despite the evidence that some supplements can be beneficial in certain circumstances, such as pregnancy or stress, horse owners should be aware that most vitamins and minerals are required in very small amounts and have specific interactions with other nutrients. Simply supplementing the diet with antioxidants, for example, is not always the best way to "boost the immune system." See TheHorse.com/5335 for a summary of these cautions.

As stated in that article, it is important to understand that vitamins and minerals in the wrong amounts can be harmful.

Fats

The revised NRC report addresses the efficacy of dietary supplements and evidence of their mechanisms of action; the chapter on fats discusses the roles of supplements such as the omega-3 fatty acids from linseed and flax, and essential fatty acids (EFAs) from fish oil. According to the report, "The data from studies in which horses were fed diets enriched with omega-3 (n-3) fatty acids (linseed, flaxseed, or fish oils) have demonstrated modulation of inflammatory mediator synthesis by cells harvested from blood, peritoneal fluid (that in the abdominal cavity), or respiratory secretions." However, the report continues, "the physiological importance of these findings is unclear and further research is needed to determine the effects of n-3 fatty acid supplementation in the treatment and prevention of inflammatory diseases in horses (e.g., recurrent airway obstruction)."

Other Factors

The effects of certain nutrients will vary, depending on a variety of factors. The NRC report breaks down these factors into five main areas: maintenance, reproduction, lactation, growth, and exercise.

The immune system functions differently during each phase of life. For example, in the foal, immunity must be established early. Antibodies essential to the foal's immune system are delivered via the mare's colostrum. Among the antibodies are proteins called immunoglobulins, which work to kill bacteria and protect against infections. The colostrum also provides vitamins, other valuable proteins, and minerals.

Unlike in human studies, where there are large populations to observe and test, along with generous research funding, equine studies are limited by variation of physiological and environmental factors in the horse population (making research difficult) and limited funding.

"Even with the funded studies, there are such small numbers that the individual variations play havoc with the results," says Crandell. "Researchers might see a change in one or more animals, but often the change is not statistically significant because of individual variation."

Take-Home Message

Through human studies and selected studies in horses, researchers have identified specific nutrients that are involved in equine immune system function. There is much to be learned about the mechanisms of nutrition and immunity; the coming years will likely shed more light on this complex field. Given the complexity of the interactions between nutrients, immunity, and specific environmental factors, horse owners should use caution in making decisions about nutritional supplements, especially when adding vitamin and mineral supplements to feed.

About the Author

Nancy Zacks holds an M.S. in Science Journalism from the Boston University College of Communication. She grew up in suburban Philadelphia where she learned to ride over fields and fences in nearby Malvern, Pa. When not writing, she enjoys riding at an eventing barn, drawing and painting horses, volunteering at a therapeutic riding program, and walking with Lilly, her black Labrador Retriever.

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